U.S. patent application number 13/297517 was filed with the patent office on 2012-05-24 for electric power supply system comprising power modules coupled in parallel.
This patent application is currently assigned to Danfoss Drives A/S. Invention is credited to Sergej Kalaschnikow, Dennis Tolstrup Kristensen, Rodney Allen Myers.
Application Number | 20120127763 13/297517 |
Document ID | / |
Family ID | 46064251 |
Filed Date | 2012-05-24 |
United States Patent
Application |
20120127763 |
Kind Code |
A1 |
Kalaschnikow; Sergej ; et
al. |
May 24, 2012 |
ELECTRIC POWER SUPPLY SYSTEM COMPRISING POWER MODULES COUPLED IN
PARALLEL
Abstract
The present invention relates to an electric power supply system
comprising at least two power modules coupled in parallel and
comprising a first power module comprising a first control means
adapted to control the operation of at least the first power
module, and a second power module comprising a second control means
adapted to control the operation of at least the second power
module and wherein the power modules are configured to operate in a
manner coordinated with other power modules, the operation of each
power module including switching the power module on and/or off if
demand so requires. The present invention further relates to a
method for carrying out the present invention.
Inventors: |
Kalaschnikow; Sergej; (Wien,
AT) ; Myers; Rodney Allen; (Loves Park, IL) ;
Kristensen; Dennis Tolstrup; (Loves Park, IL) |
Assignee: |
Danfoss Drives A/S
Graasten
DK
|
Family ID: |
46064251 |
Appl. No.: |
13/297517 |
Filed: |
November 16, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61416524 |
Nov 23, 2010 |
|
|
|
Current U.S.
Class: |
363/34 ;
363/65 |
Current CPC
Class: |
H02J 1/10 20130101 |
Class at
Publication: |
363/34 ;
363/65 |
International
Class: |
H02J 3/50 20060101
H02J003/50; H02M 5/42 20060101 H02M005/42; H02J 3/46 20060101
H02J003/46 |
Claims
1. An electric power supply system comprising at least two power
modules coupled in parallel with an associated power supply line,
the electric power supply system comprising: a first power module
adapted to supply power to an associated power consumer, the first
power module comprising a first control means adapted to control
the operation of at least the first power module, and a second
power module adapted to supply power to the associated power
consumer, the second power module comprising a second control means
adapted to control the operation of at least the second power
module, wherein the power modules are configured to operate in a
manner coordinated with other power modules, the operation of each
power module including switching the power module on and/or off if
demand so requires.
2. The electric power supply system according to claim 1, further
comprising one or more additional power modules coupled in parallel
with the associated power supply line.
3. The electric power supply system according to claim 2, wherein
the one or more additional power modules are adapted to supply
power to the associated power consumer, and wherein each of the one
or more additional power modules are configured to operate in a
manner coordinated with one or more other power modules, the
operation of each additional power module including switching the
itself on and/or off if demand so requires.
4. The electric power supply system according to claim 1, wherein
the first power module is a master power module, the master power
module comprising master control means at least adapted to control
one or more slave power modules, and the second power module is a
slave power module, the slave power module comprising a control
means adapted to communicate with the master control means of the
master power module wherein the slave power module is configured to
be operated in response to its communication with the master
control means of the master power module, the operation of the
slave power module including switching the slave power module on
and/or off if demand so requires.
5. The electric power supply system according to claim 4, further
wherein one or more of the additional power modules are additional
slave power modules.
6. The electric power supply system according to claim 5, wherein
each of the one or more slave power modules comprises control means
adapted to communicate with the master control means of the master
power module.
7. The electric power supply system according to claim 6, wherein
the one or more additional slave power modules are configured to be
operated in response to their respective communication with the
master control means of the master power module, the operation of
the one or more additional slave power modules including switching
the one or more additional slave power modules on and off if demand
so requires.
8. The electric power supply system according to claim 4, wherein
communication between master and slave power modules is provided
via a communication means, such as a data bus.
9. The electric power supply system according to claim 1, wherein a
number of the at least two power modules comprise one or more of:
an AC to DC rectifier, said AC to DC rectifier forming a front end
of a frequency converter, a DC to AC converter, said DC to AC
converter forming a rear end of a frequency converter, a frequency
converter.
10. The electric power supply system according to claim 1, wherein
a number of the at least two power modules comprise one or more of:
an active filter and a power factor correction circuit.
11. A method of operating an electric power supply system
comprising at least two power modules coupled in parallel with an
associated power supply line, the method comprising the steps of:
providing an electric power supply system comprising a first power
module adapted to supply power to an associated power consumer, the
first power module comprising a first control means adapted to
control the operation of at least the first power module, and a
second power module adapted to supply power to the associated power
consumer, the second power module comprising a second control means
adapted to control the operation of at least the second power
module, operating the power modules in a manner coordinated with
other power modules, the operation of each power module including
switching the power module on and/or off if demand so requires.
12. The method according to claim 11, wherein the electric power
supply system further comprises one or more additional power
modules coupled in parallel with the associated power supply
line.
13. The method according to claim 12, wherein the one or more
additional power modules are adapted to supply power to the
associated power consumer, and wherein each of the one or more
additional power modules are configured to operate in a manner
coordinated with one or more other power modules, the operation of
each additional power module including switching the additional
power module on and/or off if demand so requires.
14. The method according to claim 11, wherein the first power
module is a master power module, the master power module comprising
master control means at least adapted to control one or more slave
power modules, and the second power module is a slave power module,
the slave power module comprising a control means adapted to
communicate with the master control means of the master power
module and the method further comprises operating the slave power
module in accordance with its communication with the master control
means of the master power module, the operation of the slave power
module including switching the slave power module on and/or off if
demand so requires.
15. The method according to claim 14, wherein the electric power
supply system further comprises one or more additional slave power
modules coupled in parallel with the associated power supply
line.
16. The method according to claim 15, wherein the one or more
additional slave power modules are adapted to supply power to the
associated power consumer, and wherein each of the one or more
slave power modules comprises control means adapted to communicate
with the master control means of the master power module.
17. The method according to claim 16, wherein the one or more
additional slave power modules are configured to be operated in
response to their respective communication with the master control
means of the master power module, the operation of the one or more
additional slave power modules including switching the one or more
additional slave power modules on and off if demand so
requires.
18. The method according to claim 14, wherein communication between
master and slave power modules is provided via a communication
means, such as a data bus.
19. The method according to claim 11, wherein a number of the at
least two power modules comprise one or more of: an AC to DC
rectifier, said AC to DC rectifier forming a front end of a
frequency converter, a DC to AC converter, said DC to AC converter
forming a rear end of a frequency converter, a frequency
converter.
20. The method according to claim 11, wherein a number of the at
least two power modules comprise one or more of: an active filter
and a power factor correction circuit.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is entitled to the benefit of and
incorporates by reference essential subject matter disclosed in
Provisional Patent Application Ser. No. 61/416,524 filed on Nov.
23, 2010.
FIELD OF THE INVENTION
[0002] The present invention relates to a power supply system
comprising a plurality of power modules, such as power supply
modules or active filters, coupled in parallel. In particular, the
present invention relates to a power supply system to be inserted
between a power grid and a power consumer. The power grid may be a
single or a three-phase power grid. The power supply system
according to the present invention is capable of activating or
deactivating individual power modules in accordance with for
example the amount of power to be delivered to the power consumer.
By constantly adjusting the number of activated power modules the
total amount of electrical losses and the level of electromagnetic
noise generated by the power supply system may be significantly
reduced.
BACKGROUND OF THE INVENTION
[0003] Various types of modular systems, such as modular filter
systems, have been suggested in the field of power electronics. For
example, the company Asea Brown Boveri (ABB) has in the Pamphlet
"Quality Power Filter--Active Filtering Guide" disclosed how a
plurality of active filters can be arranged in a parallel
configuration. In the filter system suggested by ABB a master
active filter controls a number of slave active filters.
[0004] However, it is a disadvantage of the system disclosed by ABB
that all slave active filters are switched on at all times. By
having all slave active filters switched on in such a continuous
manner the total switching losses in the filters, and the generated
electromagnetic noise reach unnecessary high levels. Thus, when
viewed from a loss and noise perspective, the filter system
proposed by ABB can be further improved.
SUMMARY OF THE INVENTION
[0005] It may be seen as an object of the present invention to
provide a power supply system where electrical losses in the form
of conduction losses and switching losses are reduced.
[0006] It may further be seen as an object of the present invention
to provide a power supply system where electromagnetic noise, such
as switching noise from controllable semiconductor switching
elements, transmitted into an associated power grid is
significantly reduced.
[0007] It may even further be seen as an object of the present
invention to provide a redundant and thereby reliable power supply
system.
[0008] It may even still further be seen as an object of the
present invention to provide a modular power supply system where
the number of active power modules is constantly adjusted in order
to match, for example, the amount of electrical power to be
delivered.
[0009] The above mentioned objects are complied with by providing,
in a first aspect an electric power supply system comprising at
least two power modules coupled in parallel with an associated
power supply line, the electric power supply system comprising a
first power module adapted to supply power to an associated power
consumer, the first power module comprising a first control means
adapted to control the operation of at least the first power
module, and a second power module adapted to supply power to the
associated power consumer, the second power module comprising a
second control means adapted to control the operation of at least
the second power module, wherein the power modules are configured
to operate in a manner coordinated with other power modules, the
operation of each power module including switching the power module
on and/or off if demand so requires.
[0010] In a second aspect, the present invention relates to a
method of operating an electric power supply system comprising at
least two power modules coupled in parallel with an associated
power supply line, the method comprising the steps of firstly
providing an electric power supply system comprising a first power
module adapted to supply power to an associated power consumer, the
first power module comprising a first control means adapted to
control the operation of at least the first power module, and a
second power module adapted to supply power to the associated power
consumer, the second power module comprising a second control means
adapted to control the operation of at least the second power
module, and, secondly, operating the power modules in a manner
coordinated with other power modules, the operation of each power
module including switching the power module on and/or off if demand
so requires.
[0011] The terms power supply system and power module are to be
interpreted very broadly in that these terms are not to be
interpreted only as meaning power generating means involving, for
example, a generator and suitable rectifiers and/or other
converters for bringing the supplied electricity into a desired
form. As a result of the above-mentioned broad interpretation a
power module may be interpreted as, for example, a complete
frequency converter, an active rectifier, an active front end or
rear end of a frequency converter, an active filter, a power factor
correction circuit etc. All these power modules may apply
controllable semiconductor switching elements, such as thyristors,
power transistors, such as insulated gate bipolar transistors
(IGBT), for providing electric power to a power consumer in a
desired form. The supply of power to the associated power consumer
or the filtering of harmonic noise appearing on the power supply
line may be provided by means of one or more of several modulation
techniques which are well known in the art such as pulse width or
pulse amplitude modulation techniques.
[0012] Thus, according to the first and second aspects of the
present invention a power supply system comprising two or more
power modules coupled in parallel is provided. Such a configuration
provides a redundant and reliable power supply system.
[0013] It is a huge advantage of the present invention that by
constantly ensuring that only a minimum number of power modules are
active, electrical losses in the form of conduction losses and
switching losses are reduced to a minimum. Even further,
electromagnetic noise, such as switching electromagnetic noise
originating from controllable semiconductor switching elements,
transmitted into an associated power grid may be significant
reduced.
[0014] The electric power supply system may further comprise one or
more additional power modules coupled in parallel with the
associated power supply line, and adapted to supply power to the
associated power consumer, and wherein each of the one or more
additional power modules are configured to operate in a manner
coordinated with one or more other power modules, the operation of
each additional power module including switching the itself on
and/or off if demand so requires. In this way the electric power
supply system may be custom designed in order to fulfil
predetermined demands, such as maximum power to be delivered, a
specific loss or noise level to be complied with, etc.
[0015] The electric power supply system may even further comprise a
system wherein the first power module is a master power module, the
master power module comprising master control means at least
adapted to control one or more slave power modules, and the second
power module is a slave power module, the slave power module
comprising a control means adapted to communicate with the master
control means of the master power module and wherein the slave
power module is configured to be operated in response to its
communication with the master control means of the master power
module, the operation of the slave power module including switching
the slave power module on and/or off if demand so requires.
[0016] In addition, each of the one or more slave power modules may
comprise control means adapted to communicate with the master
control means of the master power module. The one or more
additional slave power modules may be configured to be operated in
response to their respective communication with the master control
means of the master power module, the operation of the one or more
additional slave power modules including switching the one or more
additional slave power modules on and/or off if demands so
requires.
[0017] The terminology master/slave power module may not
necessarily be static. Thus, a slave power module may be appointed
master power module if, for example, a previous master power module
fails or breaks down during operation. The new master power module
may be appointed manually or automatically for example by a higher
level control system. Operation without master power module is
possible as well. In this case, the higher level control system has
a master function or each power module has the ability to function
as a master power module.
[0018] The master and slave power modules may be interconnected by
a communication means such as a data or communication bus, such as
a serial data bus. In this way the master power module is capable
of controlling the operation of the slave power module, including
switching the slave power module on and/or off so that the number
of active power modules is appropriate for, for example, the amount
of power to be delivered, a specific loss or noise level to be
complied with, etc.
[0019] As previously stated, the plurality of power modules may
comprise power modules such as one or more AC to DC rectifiers,
said AC to DC rectifier forming a front end of a frequency
converter, one or more DC to AC converters, said DC to AC converter
forming a rear end of a frequency converter, one or more complete
frequency converters, one or more active filters or one or more
power factor correction circuits etc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention and its advantages will become more
apparent, when looking at the following description of possible
embodiments of the invention, which will be described with
reference to the accompanying figures wherein:
[0021] FIG. 1 shows an embodiment of the present invention
involving four active filters coupled in parallel,
[0022] FIG. 2 shows a second embodiment of the present invention
involving four active filters coupled in parallel and a means of
controlling the individual filters,
[0023] FIG. 3 shows the flow chart for a method of controlling the
filters illustrated in the second embodiment,
[0024] FIG. 4 shows a third embodiment of the invention wherein a
master/slave control method is used,
[0025] FIG. 5 shows the flow chart for the master/slave control
method described in the third embodiment,
[0026] FIG. 6 shows a fourth embodiment of the invention involving
four active filters coupled in parallel, and
[0027] FIG. 7 shows a fifth embodiment of the invention involving
four active filters coupled in parallel.
[0028] FIG. 8 shows a sixth embodiment of the invention involving
four active filters coupled in parallel.
[0029] While the invention is susceptible to various modifications
and alternative forms, a specific embodiment have been shown by way
of example in the figures and will be described in detail herein.
It should be understood, however, that the invention is not
intended to be limited to the particular forms disclosed. Rather,
the invention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
[0030] In its most general form, the present invention relates to a
power supply system comprising two or more power modules coupled in
parallel and thereby establishing a redundant and reliable power
supply system. The power modules are controlled in a way which
activates only the number of power modules which is appropriate for
the amount of power to be delivered. By constantly ensuring that
only a minimum number of power modules are active, electrical
losses in the form of conduction losses and switching losses are
reduced to a minimum. Even further, electromagnetic noise, such as
switching noise from controllable semiconductor switching elements,
emitted into an associated power grid may be significantly reduced.
Various types of semiconductor switching elements are applicable.
However, semiconductor switching elements such as thyristors, power
transistors, such as IGBTs, are the most common type of
semiconductor switching elements.
[0031] So far the present invention has been disclosed with
reference to applying a plurality of power modules in a parallel
configuration. However, the principle of the present invention also
applies to, for example, active filters or power factor correction
circuits coupled in parallel.
[0032] FIG. 1 shows an embodiment of the present invention
involving, for example, four active filters coupled in parallel.
However, the present invention is not limited to systems involving
only active filters coupled in parallel. Thus, the active filters
shown in FIG. 1 are only exemplary and could be replaced by
complete frequency converters, active front ends of a frequency
converter etc. without departing from the scope of the present
invention.
[0033] As depicted in FIG. 1 the four active filters 7, 8, 9, 10
are inserted between a supply grid 1 and a power consumer 15. The
harmonics or noise generated by the consumer, and consequently the
work required from the active filters, varies with time, as is well
known in the art. An optional grid transformer 2 has been inserted
between the power grid 1 and the leads 3, 4, 5, 6 connected to
respective ones of filters 7, 8, 9, 10.
[0034] Each of the active filters is individually capable of
compensating for some of the harmonic noise power generated by the
nonlinear power consumer, but not the maximum possible harmonic
noise power. It is for this reason that four are connected in
parallel in this embodiment. In prior art systems, as described
above, this has the disadvantage of increased switching losses over
a system with a single, larger capacity, active filter. However, in
this embodiment of the invention, the four active filters are
controlled in a manner in which only the appropriate number of
active filters necessary for the amount of power to be delivered
are switched on. For example, when the power required from the
active filtering modules is a maximum, all the active filters are
switched on and when only half the power from the active filtering
modules is required, filters 9 and 10 are turned off, and filters 7
and 8 remain on. By constantly ensuring that only a minimum number
of power modules are active, electrical losses in the form of
conduction losses and switching losses are reduced to a minimum.
Even further, electromagnetic noise, such as switching noise from
controllable semiconductor switching elements, launched into an
associated supply grid may be significantly reduced.
[0035] FIG. 2 shows a second embodiment of the present invention
involving, for example, four active filters coupled in parallel. As
described above, other types of module may be used. FIG. 2 also
illustrates a means of controlling the individual filters.
[0036] As depicted in FIG. 2, the four active filters 7, 8, 9, 10
are inserted between a supply grid 1 and a power consumer 15. An
optional grid transformer 2 has been inserted between the power
grid 1 and the leads 3, 4, 5, 6 connected to respective ones of
filters 7, 8, 9, 10.
[0037] The active filters 7, 8, 9, 10 are interconnected by several
binary communication lines 16, 17, 18, 19 and by this means the
active filters 7, 8, 9, 10 may communicate with each other and
decide whether to be in `Run` (`Active`, `Turned on` or `Alive`)
mode or `Stop` (`Inactive`, `Sleep`, `Stop` or `Turned off`) mode
so that the number of active filters 7, 8, 9, 10 in `Run` mode is
maintained as appropriate for the amount of power to be delivered.
By constantly ensuring that only a minimum number of active filters
7, 8, 9, 10 are in `Run` mode, the advantages described above may
be attained.
[0038] The binary communication line 16 conducts signals from the
active filter 7 to the other active filters 8, 9, 10. Such a signal
may be a high voltage (or `1`) representing the fact that the
active filter 7 is in `Run` mode and a low voltage (or `0`)
representing the fact that the active filter 7 is in `Stop` mode
and is turned off, or any other means of communication well known
in the art. The binary communication lines 17, 18 and 19 connect
the other active filters 8, 9, 10 in a respective manner.
[0039] In this embodiment, all the active filters 7, 8, 9, 10 need
to know or monitor the total system load. This is accomplished by
use of a load or current measuring device 27 and the communication
line 28 which supplies the load information to the individual
active filters 7, 8, 9, 10. Since the active filters 7, 8, 9, are
able to exchange information about their present modes by using the
binary communication lines 16, 17, 18, 19 as described above, they
are therefore able to control themselves without any external
command. The control method will now be described.
[0040] Assuming that the power rating of each active filter 7, 8,
9, 10 has the same value, the reference R for output is calculated
by each active filter using following formula:
R = L m ##EQU00001##
where L is the present value of the total system load (obtained
from the load or current measuring device 27 and supplied to the
individual active filters 7, 8, 9, 10 via the communication line
28), and m is the number of active filters currently in `Run` mode.
The value of m is available to an individual active filter 7, 8, 9,
10 via the binary communication lines 16, 17, 18, 19. When in `Run`
mode, an individual active filter 7, 8, 9, 10 will reduce the
harmonic noise content of the supply grid 1 by using up to a
maximum power of R.
[0041] Assuming that the power rating of each active filter 7, 8,
9, 10 has the same value and each active filter is designated a
number (n.sub.u) from 1 to N (N being the total number of active
filters) then the condition for each active filter change its mode
to `Stop` mode is:
L < ( P N ( n u - 1 ) - h ) ( 1 ) ##EQU00002##
where P is the sum power capacity of all the active filters
connected in parallel and is available to each active filter by
being pre-programmed, N is the number of active filter connected in
parallel and is available to each active filter by being
pre-programmed, n.sub.u is the designated number of the particular
active filter (n.sub.u=1, 2 . . . N) and h is a hysteresis value.
Hysteresis can be used to filter signals so that the output reacts
slowly by taking recent history into account.
[0042] The corresponding condition for a particular active filter
to return to `Run` mode is:
L > ( P N ( n u - 1 ) + h ) ( 2 ) ##EQU00003##
[0043] The conditions (1) and (2) are continuously calculated by
each active filter.
[0044] FIG. 3 shows the flow chart for the method described above.
At startup 20 the pre-programmed values for P, N, n.sub.u and h are
read. The value of the reference R is calculated at 21 using the
value of the present load L, obtained from the load or current
measuring device 27. At the decision point 22 a jump is made to
decision point 25, if the active filter is not in `Run` mode,
otherwise a decision is made at 23 depending upon the result of the
condition (1) described above. If negative, that is if L is greater
than
( P N ( n u - 1 ) - h ) , ##EQU00004##
then the control passes to a recalculation of R at 21. If positive,
then the active filter will go into `Stop` mode at 24 and then
calculate condition (2) at 25 to determine whether to return to
`Run` mode again.
[0045] FIG. 4 illustrates a third embodiment of the invention
wherein a master/slave control method is used, wherein one of the
modules acts as a master power module whereas the other power
module or modules act(s) as (a) slave power module(s). The power
modules are interconnected by a communication means such as, for
example, a serial data bus. In this way the master power module is
capable of controlling the operation of the one or more slave power
modules, including switching the slave power modules on and/or off
so that the number of active power modules is appropriate for the
amount of power to be delivered, with the resultant advantages that
are described above.
[0046] With use of serial communication there is, in principle, no
limitation to the number of control variables passed between the
modules. The master power module can pass a control word containing
such discrete values as `Start`, `Stop`, and `Standby`. The slave
power module can respond with a status word containing such
discrete values as warnings and alarms. This can be utilized by the
master power module to determine and set which of the slave power
modules performs the majority of the work to be done.
[0047] FIG. 4 shows, for example, four active filters 29, 30, 31 32
coupled in parallel. The four active filters 29, 30, 31, 32 are
inserted between a supply grid 1 and a power consumer 15. An
optional grid transformer 2 has been inserted between the power
grid 1 and the leads 3, 4, 5, 6 connected to respective ones of the
active filters 29, 30, 31, 32. One of the active filters, say
active filter 29, is operated as a master active filter whereas the
remaining active filters 30, 31, 32, are operated as slave active
filters.
[0048] A communication means 11, such as, for example, a serial
data bus, ensures appropriate communication between the active
filters 29, 30, 31, 32, in particular between the master active
filter 29 and the three slave active filters 30, 31, 32. Via this
communication means 11 the master active filter 29, which may be
controlled by a higher level control system, may control the
operation parameters of all the slave active filters 30, 31, 32.
Such operation parameters may, among other things, involve
switching the slave active filters 30, 31, 32 on and/or off if
demands so requires, or it may involve that a given slave active
filter or a group of slave active filters should be operated in
accordance with a predetermined set of operation parameters, such
as a predetermined harmonic noise level, an amount of power to be
delivered, the quality of the electrical power to be delivered
etc.
[0049] Thus, it is an advantage of the present invention that the
master active filter 29 may control the number of active slave
filters 30, 31, 32 so that only the required number of slave
filters are active, thus, achieving superior performance in terms
of minimal losses and minimal noise generation.
[0050] The terminology master/slave may not necessarily be static.
Thus, a slave filter may be appointed a new master filter if, for
example, a previously appointed master filter fails or is taken
offline. The new master filter may be appointed manually or
automatically, for example by the higher level control system.
[0051] The number of master and slave filters may obviously differ
from what is depicted in FIG. 4. Thus, a plurality of master
filters may be provided. Similarly, the number of slave filters may
differ from three as depicted in FIG. 4.
[0052] FIG. 5 shows the flow chart for the master/slave control
method described in the third embodiment and illustrates the
logical process followed by the control system of the master filter
29. The slave filters 30 31 32 are required in the embodiment
merely to respond appropriately to `Run` and `Stop` mode commends
from the master active filter 29, and to report relevant warnings
and status messages.
[0053] At startup 33 the pre-programmed values for P, N, m and h
are read. The present value of the reference R is calculated at 34
using the value of the present load L, obtained from the load or
current measuring device 27 and m, the number of active filters
currently in `Run` mode. At the decision point 35 the equation (1)
described above is calculated for the first active filter
(n.sub.u=1) and a decision is made as to whether to place the first
active filter into `Stop` mode 36, or to continue directly to a
recalculation of the reference R at 37. At 38 equation (1) is
calculated for the second active filter (n.sub.u=2) and a decision
is made as to whether to place the second active filter into `Stop`
mode 39, or to continue directly to a recalculation of the
reference R. This process of calculating the current reference R,
calculating equation (1) and stopping the respective filter
continues until the last filter (n.sub.u=N) is reached 40, 41, 42.
A similar sequence is then performed using equation (2) this time,
to assess whether the respective filters should be placed in `Run`
mode is made 43-51 and the sequence finally returns to a
recalculation of R at 34.
[0054] FIG. 6 shows a fourth embodiment of the invention involving
four active filters coupled in parallel. The four active filters 7,
8, 9, 10 are connected to a supply grid 1 which also supplies a
power consumer 52. The harmonic noise generated by the consumer,
and consequently the work required from the active filters, varies
with time, as is well known in the art. As described in the other
embodiments above, each of the active filters is individually
capable of compensating for some of the harmonic noise power
generated by the nonlinear power consumer, but not the maximum
possible harmonic noise power. As described above, the four active
filters are controlled in a manner in which only the appropriate
number of active filters necessary for the amount of power to be
delivered to compensate for the harmonic noise generated are
switched on. In this embodiment, all the active filters 7, 8, 9, 10
need to know or monitor the total system load. This is accomplished
by use of a load or current measuring device 27 and the
communication line 28 which supplies the load information to the
individual active filters 7, 8, 9, 10.
[0055] FIG. 7 shows a fifth embodiment of the invention involving
four active filters coupled in parallel. This is similar to the
embodiment illustrated in FIG. 6, but with the load or current
measuring device 27 placed differently.
[0056] FIG. 8 shows a sixth embodiment of the invention involving
four active filters coupled in parallel. This is similar to the
embodiment illustrated in FIG. 6, but with the addition of a second
load 15 which is supplied with power by the four parallel active
filters 7, 8, 9, 10. Such an embodiment illustrates the fact that
such active filters can simultaneously compensate for harmonic
noise generated by loads connected as 52 or 15.
[0057] Although various embodiments of the present invention have
been described and shown, the invention is not restricted thereto,
but may also be embodied in other ways within the scope of the
subject-matter defined in the following claims.
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